The present invention relates to a current switch for monitoring electric current in a conductor and enabling an alarm if the magnitude of the current is abnormal and, more particularly, to an automatically calibrated current switch.
Industrial and commercial environments utilize large numbers of electrically powered devices, such as fans, pumps and compressors. These devices are commonly driven by alternating current (AC) induction motors which are controlled by variable frequency drives (VFD). The speed of an induction motor is substantially proportional to the frequency of the input current and the basic function of the VFD is to act as a variable frequency generator to vary the speed of the motor in response to commands from a controller. The operation of these motor driven devices can be important to the protection of valuable property and successful completion of processes that may involve costly or hazardous equipment, operations or materials. The operation of these devices is commonly monitored by a current transducer that is electromagnetically coupled to a cable supplying power to the motor. The current transducer outputs a signal that is representative of the magnitude of current flowing in the cable and, if the current changes in a significant manner, the transducer transmits an alarm to the controller which may display a warning or an advisory signal on a control panel for a human operator and/or selectively enable or disable power to the device and/or other devices of a system that may be affected by a malfunction of the monitored device.
Hunter et al., U.S. Pat. No. 7,855,655 B2, discloses a current switch for monitoring a motor controlled by a VFD. To calibrate the current switch, the VFD is directed to supply a current to a motor at a frequency within each of a plurality of frequency bands. After the motor has reached steady state operation, the current switch accumulates a number of current samples, averages the magnitudes of the current samples and stores the average value as the expected motor current for that frequency band, a process that is repeated for each of the plurality of frequency bands. Thereafter, the current switch will initiate an alarm if the current differs from the average current for a frequency band by more than a predetermined percentage. While the calibration process is automatic and straight forward, it is time consuming because the VFD must be operated for approximately a minute at a frequency in each frequency band to allow the motor to reach steady state and then allow the accumulation of a number of current samples. If the installation includes a large number of current switches, the calibration process can be lengthy and there is the possibility that the installer may omit one or more frequency bands during the calibration process.
What is desired, therefore, is a current switch for monitoring the operation of a VFD controlled device which is automatically calibrated while the device is operating.